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Trouble Shooter

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Trouble Shooter
Just about the time you think you understand something,
someone comes along to challenge your previously held
assumptions. It can be confusing, but that’s usually a good thing.
Hollow Log
[email protected]
I received several responses to my request for
additional input at the end of my May column.
To refresh your memory, the problem involved a
2000 Chevy Silverado 1500 pickup with about
185,000 miles on it. The Check Engine light was
on and a DTC P0137 (bank 1 sensor 2 low voltage) was stored in memory.
With the engine idling, bank 1 sensor 2 produced a steady 40mV, and sensor output voltage
fluctuated slightly on acceleration. The bank 2
sensor 2 output voltage fluctuated normally.
Swapping the harness connectors between bank
1 sensor 2 and bank 2 sensor 2 resulted in a bank
2 sensor 2 fault, and the PCM set another DTC
after the second drive cycle.
The exhaust gas readings at idle were .1% CO
and 75 ppm HC. At cruise, the readings were
.2% CO and 42 ppm HC. Manifold vacuum
readings were 18 in. at idle and 19 in. at cruise.
Next, the bank 1 sensor 2 harness connector
was unplugged, then a new sensor was plugged
into the harness without installing it in the exhaust. Reading ambient air only, the new sensor
output was higher than when installed and the
engine running. Soon afterward, it was discovered that the bank 1 converter was empty. After
installing a new converter and oxygen sensor,
bank 1 sensor 2 switched rich/lean/rich as it
should and no further DTCs were stored.
Several readers suggested that some mischief
must have taken place before the truck was
brought into the shop. After all, converters don’t
normally shed their internals unless they’ve been
severely overheated. And even then, some evidence of the converter monolith should have
been found downstream; possibly bits and pieces
of it should have been plugging the muffler.
Tom’s customer, the owner, had only recently
purchased the truck, and indicated that the
Check Engine light was lit at that time, so he’s
probably blameless. Perhaps the previous owner
did the deed.
Reader Steve Petry suggested that the sensor’s
original low, fixed output at idle may have been
caused by a damaged sensor heater. Without a
heater, the sensor gets warm enough to operate
only when there’s increased exhaust flow. This
matches Tom’s description of the original sensor’s operation during acceleration. The heater
may have been damaged at the same time the
converter was overheated and damaged. But
why was the converter on only one bank damaged? A severe bank 1 misfire? Tom indicated
Photo: Karl Seyfert
continued on page 8
Conventional oxygen sensors like this one do a pretty good job of indicating an engine’s
air/fuel ratio. They’re most accurate when the engine is close to stoichiometry and less accurate at the rich and lean ends of the spectrum, which is why they’re being replaced.
July 2010
Trouble Shooter
he had checked the bank 1 sensor 2
heater circuit; perhaps he missed
something. The heater should have
been resistance and current draw tested, then compared to the bank 2 sensor
2 heater.
Reader Bob Russo wrote to say he’d
recently worked on a Toyota truck that
was storing a DTC P0420 (low cat efficiency). The precat oxygen sensor was
cycling normally and the postcat sensor
was showing a low, relatively constant
signal of around 250mV. He tried
everything, including an oxygen sensor
replacement, but the code would always reappear. When he removed the
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converter, it was gutted, just like Tom’s
customer’s. The owner stated that he
had been driving the truck for months
before the code showed up. After Bob
replaced the converter, all was fine.
This example doesn’t help us explain
Tom’s problem; it only indicates that
similar things can happen when the converter is empty. Bob replaced the rear
sensor before installing a new converter,
so a heater problem can probably be
ruled out. Why didn’t the rear sensor
output track the front one? Bob chalked
it up to another weird day in the shop.
The last note I’ll share came from
Fred Wilson, who asks us to think of
oxygen sensors differently than we may
be accustomed. In my previous column, I stated that low oxygen content
= high oxygen sensor voltage reading =
rich fuel mixture, and high oxygen content = low oxygen sensor voltage reading = lean fuel mixture. This is the way
I’ve been taught to understand oxygen
sensor function. And when a catalytic
converter is functioning properly, I’d also been taught that most of the available oxygen is absorbed by the converter, leaving very little to be measured by
the postcat sensor.
Here’s Fred’s response:
“These statements imply that the
sensor measures exhaust gas oxygen
content. We have been taught this concept for years. But it does not hold up
to careful examination of how the sensor works. The sensor catalyzes excess
hydrogen in the exhaust stream. It catalyzes the hydrogen using oxygen from
the exhaust gases. If there is excess hydrogen and low oxygen content (a rich
mixture), the sensor draws additional
oxygen from the outside air. This outside oxygen flow is what the sensor is
sensing. The more of this oxygen that is
needed, the higher the sensor voltage
that is produced, up to a limit of about
1 volt.”
Thanks to all M OTOR readers for
your responses. This discussion points
up the limitations of the conventional
oxygen sensor, and makes it easier to
understand why a more responsive and
accurate replacement was needed.
For more on that, refer to Sam Bell’s
article on air/fuel sensors (the oxygen
sensor’s replacement), on page 28.
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